Photodynamic monitor for inspecting spun yarns



H. c. LINDEMANN ETAL 3,174,046

March 16, 1965 PHOTODYNAMIC MONITOR FOR INSPECTING SPUN YARNS 4 Sheets-Sheet 2 Filed Sept. 5, 1961 INVENTOR Cv LlNDEMANN MINDHE HOWARD DANIEL lr l.

March 1965 H. c. LINDEMANN ETAL 74,

PHOTODYNAMIC MONITOR FOR INSPECTING SPUN YARNS Filed Sept. 5. 1961 4 Sheets-Sheet 3 INVENTORS HOWARD c. LINDEMANN DAN'EL mmouem United States Patent 3,174,046 PHOTODYNAMIC MONITOR FOR INSPECTING SPUN YARNS Howard C. Lindemann, Westhury, and Daniel Mindheim, Glen Cove, N.Y., assignors to Lindly & Company, Inc, Nassau (Iounty, N.Y., a corporation of New York Filed Sept. 5, 1961, Ser. No. 136,111 2 Claims. (Cl. 250-219) This is a continuation-in-part of our application, Serial No. 738,074, filed May 27, 1958, now Patent No. 3,041,461 patented June 26, 1962.

The present invention relates to photodynamic monitor apparatus for photoelectrically inspecting moving sheet material. The term sheet material is used in a generic sense to cover sheets of all kinds including sheets of yarns, threads, fibers or other strands which are not interconnected but extend parallel to one another in substantially the same plane and includes interconnected fibers or yarns and the like forming a fabric.

The invention is particularly applicable to the textile industry Where spun yarns or other fibers strands are fed in the form of sheets to various machines. For example, a sheet of Warp yams is fed to a loom. In order to assure proper operation of the machine, it is necessary to provide means for detecting any break that occurs in any of the yarns. Heretofore it has been customary to run each yarn through a drop-wire connected to suitable indicating or stop-motion mechanism. However, this has the disadvantage that the drop-Wires have to be threaded individually, add tension to the yarn, tend to abrade the yarns and do not detect a break until the broken end of the yarn passes the drop-wire.

It is an object of the present invention to provide an improved inspecting apparatus having important advan' tages over the equipment heretofore available.

Briefly, in accordance with a general aspect of the invention, apparatus is provided for photoelectrically inspecting a moving sheet by two beams of luminous flux or two portions of a beam of light, directed across a working zone of the apparatus in which the sheet is disposed and through which it moves. One beam portion is directed transversely across one face of the sheet and a second beam portion is directed transversely across the opposite face of the sheet. The beams are positioned in close proximity to the faces of the sheet so that any deviation from the normal of either face interrupts one of both of the light beams. Means sensitive to the light of one or both of the light beams is provided to detect any diminution or" the amount light in the light beams.

In one specific embodiment of the invention the moving sheet is inspected photoelectrically by a beam of light which is directed transversely across one face of the sheet and then reversed and directed transversely across the opposite face of the sheet so that both faces are inspected by successive portions of a single beam of light which has the dual role of inspecting both faces. Thus, the light transmitting and the light receiving means are positioned on the same edge of the sheet.

In another embodiment of the invention two beams of light are directed transversely across the opposite faces of the sheet from a single light source located adjacent one side edge of the sheet. A light detection means is positioned at the opposite edge of the sheet to detect decreases in the intensity of the light in either or both beams. Thus, the light transmitting and receiving means are positioned at opposite edges of the sheet.

As applied in inspecting sheets of yarns, the apparatus in accordance with either embodiment of the invention has the important advantage that it can detect a break in a yarn before the broken end reaches the yarn proc- "ice essing apparatus. When a yarn breaks, the sudden release of tension in the yarn generates a wave which travels along the yarn in somewhat the same manner as a ripple produced by throwing a stone in a body of water. When the wave reaches the inspecting station, it momentarily intercepts the light of the beam on one or both sides of the sheet and is thereby detected. As compared with conventional drop-wires, the apparatus of the invention has the further advantages that it does not require individual threading of the yarns, adds no drag to the yarn and is more sensitive and reliable in operation.

A particularly important application of the invention is with respect to detecting wild yarn or parasite yarn" in sheets of spun yarn ends. In spinning operations a considerable amount of flutt is picked up and spun into a yarn resulting in a defect. Moreover, fluff may be picked up by yarn ends and Will show up as a defect in the fabric made from the yarn ends. This type of defect will occur only with spun yarn.

This type of detection is different from detecting the so-called fluff ball which consists of a small twisted mass in the yarn ends resulting from a break of one or more monofilaments at one point, or the collection of substantial lengths of such broken monofilaments at one point in the yarn. Wild yarn" is particularly hard to detect with photoelectric apparatus since it is hairy and will allow a beam to substantially read through it. According to the invention two light beams are disposed on opposite sides of the sheet of yarn substantially in a common plane so that if asymmetrical Wild yarn" relative to the sheet is relatively dense one light beam alone will have the light therein diminished sufiiciently to detect the parasitic yarn and preclude the formation of a defect. On the other hand, wild yarn symmetrical relative to the sheet but not dense can be sensed by having the light in both beams diminished. The spacing of the beams in a common plane will also gauge the extent of wild yarn detectable by the apparatus according to the invention.

The characteristics and advantages of the apparatus in accordance with the invention will be more fully understood from the following description and claims in conjunction With the accompanying drawings in which two embodiments of the apparatus are illustrated by way of example. In the drawings:

FIG. 1 is an elevation of apparatus in accordance with one embodiment of the invention, with portions shown in vertical sections;

FIG. 2 is a horizontal section on a larger scale, the section being taken approximately on the line 22 in FIG. 1;

FIG. 3 is a vertical section taken approximately on the line 3-3 in FIG. 2;

FIG. 4 is an enlarged fragmentary elevation as viewed approximately from the line 4-4 in FIG. 1;

FIG. 5 is a section taken approximately on the line 5-5 in FIG. 4;

FIG. 6 is a section taken approximately on the line 6-6 in FIG. 4;

FIG. '7 is a section taken approximately on the line 7-7 in FIG. 4;

FIG. 8 is an elevation of the apparatus with portions shown in vertical section in accordance with another embodiment of the invention;

FIG. 9 is a vertical section of the light transmitting unit of the apparatus of FIG. 8;

PEG. 10 is a vertical section of the light receiving unit of the apparatus of FIG. 8; and

PEG. 11 is an elevation of the light transmitting unit as viewed from the edge of the sheet being inspected by the apparatus of FIG. 8.

Apparatus for inspecting a moving sheet of material in accordance with a first embodiment of the invention comprises a support or frame, light transmitting means mounted on the support adjacent one side edge of the sheet outside of a working zone in which the sheet moves and adapted to direct a beam of light transversely across one face of the sheet approximately parallel to and close to the sheet, light reversing means mounted on the support adjacent the opposite side edge of the sheet outside of the working zone and comprising means for intercepting the light beam and redirecting it transversely across the opposite face of the sheet and light sensing means mounted on the support adjacent the light transmitting means in position to receive the redirected light beam. As the light transmitting and light receiving or sensing elements are located at the same side of the sheet they are conveniently combined into a single unit.

As illustrated by way of example in the drawings, the apparatus comprises a frame or support 1, a light transmitting and receiving unit 2 mounted on the support adjacent one side edge of a sheet of material S that is to be inspected and a light reversing or redirecting unit 3 mounted on the support adjacent the opposite side edge of the sheet S. The sheet S moves through a working zone disposed between the light transmitting unit 2 and redirecting unit 3. The support is shown as comprising an elongated tubular member 5 which is preferably of rectangular cross section and which is supported adjacent its opposite ends by pairs of cross bars 6 which are secured to the tubular member. The upper bar of each pair of the cross bars 6 is supported at its opposite ends by shock mounts 7. Threaded stems 8, support the lower cross-bars 6 and engage lock nuts 9 on a stand 10. By manipulation of the nuts 9 the tubular member 5 can be accurately positioned with respect to the sheet S, which in known manner is positioned by running over a suitable bar or other guide. The support further comprises end members 11 and 12 which engage opposite ends of the tubular member 5 and are secured firmly in place by an elongated bolt 13 which extends through the tubular member 5 and both of the end members 11 and 12. As will be seen in FIG. 1, the end member 11 supports the transmitting and receiving unit 2 and the end member 12 supports the light reversing unit 3.

' The transmitting and receiving unit 2 comprises a housing 15 secured to the end member 11 by a stud bolt 16 in such manner that the housing is tiltable about the axis of the bolt 16 when the bolt is loosened and is then held in selected position by tightening the bolt. The housing 15 contains a light transmitting element shown as comprising a lamp 17 held by a base 18 which is adjustably mounted in the housing by means of adjustment screws 19 (FIG. 2). The screws 19 are adjustable to position the lamp or the filament thereof which is the source of light accurately in alignment with the optical axis of a tubular light guide 20 which opens in a window 21 in the housing 15 and contains a condensing lens 22. The lamp and light guide are positioned so as to direct a beam of light B transversely across one face of a sheet of material S that is to be inspec'ted, the light beam being substantially parallel to the sheet. The light beam B is reversed and redirected by the reversing unit 3-desribed more fully below-so as to be directed back toward the transmitting and receiving unit 2 but on the opposite side of the sheet S. The outgoing portion of the beam is designated B1 while the return portion is designated B2 (FIG. 1). The returning beam B2 enters a window 24 in the housing 15 and'is directed by a tubular light guide 25 onto a light sensing element shown as comprising a photo-tube 27. The receiving light guide 25 is parallel to the transmittinglight guide 20 but is offset from it both vertically and laterally as clearly shown in FIGS. 1, 2 and 3.

The'reversing unit 3 comprises a prism 30 (FIG. 5) having faces 31, 32 and 33. The prism is received in a box-shaped housing 35 having a peripheral flange portion 36 and a removable cover 37 secured to the housing by a plurality of screws 38 and 39 (FIG. 4). The prism 30 is positioned in the housing with its face 31 facing forwardly and engaged by the cover 37. The cover 37 is provided with two circular Windows 40 and 41 which are approximately the same size and are spaced apart the same distance as windows 21 and 24 of the transmitting .and receiving housing 15. The prism housing 35 is received in a recess 42 of a base 43 and is adjustably mounted on the base by the Allen-head screws 44, 45, 46 and 47. Screws 44, 45 and 46 extend through untapped holes in the flange portion 36 of the housing 35 and screw into tapped holes in the base 43. Screw 47 is screwed into a tapped hole in the flange portion of the housing 35 and its inner end bears against the base 43. Screw 39, which is one of the screws securing the cover 37 on the housing, extends through the flange portion of the housing and its inner end engages the base 43 so as to provide a fulcrum midway between the screws 44 and 45. Hence, by loosening screw 44 and tightening screw 45 the prism housing 45 can be tilted relative to the base 43 about an axis defined by the inner ends of screws 39 and 47. By loosening screw 47 and tightening screw 46 the housing 35 can be tilted relative to the base 43 about an axis defined by screws 44, 39 and 45. It will thus be seen that the housing is tiltable about mutually perpendicular axes and is thus capable of universal adjustment relative to the base.

The base 43 is mounted on the end member 12 of the sup port by a stud bolt 49 which is similar to and coaxial with the stud bolt 16 by means of which the housing 15 of the transmitting and receiving unit 2 is mounted on the end member 11 of the support. When the stud bolt 49 is loosened, the base 43 together with the prism and prism housing are rotatably adjustable about the axis of the bolt 49 which is then tightened to hold the base and prism in selected position.

The prism mounting base 43 is positioned on the support 12 so that a line joining the windows 40 and 41 in the cover of the prism housing is parallel to a line joining the 'indows 21 and 24 in the housing 15 of the transmitting and receiving unit 2. The outgoing portion B1 of the beam of light transmitted from the lamp 17 is received through the window 41 in thecover of the prism housing, enters the prism through the front face 31 which is disposed perpendicular to the beam, is reflected successively by prism faces 32, 31 and 33 and then emitted through the window 41 of the prism housing. The faces 32 and 33 of the prism are disposed at an angle of less than 45 degrees to the front face 31 and must be silvered. The front face 31 need not be silvered as the angle of incidence is greater than the critical angle. By adjusting the screws 44, 45, 46 and 47 the prism 30 is accurately positioned so that the return portion B2 of the beam reflected by the prisms is directed into the window 24 and light guide 25 of the light sensing element27. The light guide 25 receives only the parallel rays of the return beam B2 and substantially excludes from the phototube 27 any stray light rays. By rotating the housing 15 of the transmitting and receiving unit 2 and the base 43 of the reversing unit 3 about the axis of stud bolt 16 and 49 the outgoing portion B1 of the light beam and the return portion B2 can be positioned closer to or farther from the sheet of material S as desired. The members 8 of the support 5 provide further means of positioning the light beam accurately relative to the sheets.

From the foregoing description it will be seen that under normal conditions the outgoing portion B1 of the light beam B is transmitted transversely across one face of the sheet of material S being inspected. The light beam is received and reversed by the reversing unit 3, the return portion B2 of the beam being directed transversely across the opposite face of the sheet S and being received by the light sensing element 27. Any stray portion or fiber projecting upwardly from the sheet S will intercept at least a portion of the light of the outgoing beam B1 and in like manner any downward projection will intercept at least a portion of the return beam B2. For example, if the apparatus is being used to inspect a sheet of yarns and one of the yarns breaks, the resulting wave a propogated along the yarn will cause the yarn to vibrate so that it intercepts one or both portions of the light beam B. In the event that the propagated wave does not have sutficient amplitude, for example where crowded yarns restrain the development of the wave, the broken end of the yarn Will break one or both of the light beams. Any interception of either the outgoing or return portion of the beam causes a decrease in the light falling on the phototube 27, thereby producing a signal which may be utilized in any desired manner, for example, to notify an attendant or to actuate stop-motion mechanism.

As illustrated in FIGS. 2 and 3 the housing 15 of the transmitting and receiving unit further accommodate an amplifier 50 comprising tubes 51, 52 and 53. The phototube 27 is suitably connected to the input of the amplifier 50. A light meter 54 mounted in the housing 15 is connected to the amplifier circuit so as to give a visual indication of the amplified output of the phototube 27. The housing 15 is provided with suitable connectors or fittings 55 and 56 for making connections to external circuits for supplying power to the lamp 17 and the amplifier t and also to circuits utilizing a signal from the phototube 27, for example, to actuate stop-motion mechanism.

It will thus be seen that this embodiment of the invention provides simple and effective apparatus for scanning both sides of moving sheet material by successive portions of a single light beam.

In detection of parasitic yarn the two beams must be substantially in a common plane. This allows the parasitic yarn to enter both beams at the same time and will thus cause the signal developed to build up to a maximum in the shortest possible time thereby allowing the sensing the least hairiest parasitic yarn. Furthermore, developing the signal in the shortest possible time results in a higher frequency which is more easily amplified.

Apparatus for inspecting a moving sheet of material in accordance with a second embodiment of the invention includes a support for frame and light transmitting means mounted on the support adjacent to one side edge of the sheet and adapted to direct a pair of beams of light transversely across the two faces of the sheet. The beams of light are approximately parallel to and in close proximi ty to the sheet. Sensing means mounted on the support are operatively positioned at the other side edge of the sheet to receive the light beams. The sensing -means includes a light intensity detecting element to detect the interception of either light beam by portions of the sheet.

As illustrated by way of example, FIG. 8 shows the apparatus comprising a frame or support 1', a light transmitting unit 2, mounted on the support outside of the working zone adjacent one side of a sheet of material S that is to be inspected, and a light receiving unit 3 mounted on the support outside of the working zone adjacent the opposite side edge of the sheet S. The support is shown as comprising an elongated tubular member 5, which is preferably of rectangular cross-section and is supported adjacent its opposite ends by pairs of cross bars 6 which are secured to the tubular member. The upper bar of each pair of the cross bars 6 is supported at its ends by shock mounts 7'. Threaded stems 8 supports the lower cross bars 6 and engage lock nuts 9' on a stand 10.

By manipulating the nuts 9, the tubular member 5 can be accurately positioned with respect to the sheet S which in known manner is positioned by running over a suitable bar or frame. It should be noted that although the sheet S is shown in a horizontal position, the apparatus is equally suitable for inspecting vertically positioned sheets provided the same relative orientation is maintained between the light beams and the sheet 8'. The respective base members 10? and 111 of the light transmitting unit and the light receiving unit rest on opposite ends of the tubular member 5 and are secured firmly in place by bolts 113 which extend through the tubular member 5 and the base members 1% and 111. Although only two *5 bolts 113 are shown associated with each base member there are actually four to insure positive engagement and stability.

During operation, the light transmitting unit 2 transmits a light beam B1 transversely across one face of the sheet S and a second beam of light B2 transversely across the opposite face of the sheet S. The beams of light B1 and B2 are each projected parallel to and in close proximity to the faces of the sheet S. The light receiving unit 3 is positioned to receive these beams of light. Thus, whenever a yarn in the sheet is broken, either one or the other or both of the light beams B1 or B2 is intercepted and there is a momentary decrease in the amount of the light received by the light receiving unit 3. In particular, light from a light source 10% in the light transmitting unit 2 is formed into a parallel beam by a collimating means 1G2 and divided into two spaced parallel beams by the apertures ltlda and 10%. The parallel beams thus formed are accepted by the apertures 106a and 1061) in the light receiving unit 3'. These two beams are converged by a beam converging means 108 into a single beam in the plane of a slit as hereafter explained. The amount of the transmitted light is monitored by a light detection means 119. Whenever there is a decrease in the amount of this light because of the interception of either one of the beams B1 or E2, the light detection means 110 can transmit a signal.

FIG. 9 shows the light transmitting unit 2 in greater detail. A housing 112 which rests on base member 169 contains a li ht source 104 and provides support for the collimating means 102. The light source 160 includes a vacuum tube 100:: and high frequency coil 10912 to pro vide a source of high frequency electrical energy to a lamp 10%. The collimating means 1tl2, which is shown as comprising a hollow cylinder or tube 126 containing a diaphragm 114 and a condensing lens 116 mounted in a retainer 117 forms a beam of light which is parallel to the axis of the collimating means. A pair of slits or apertures 184a and 164]; in a cap or lens hood 118 divides this parallel beam into a pair of thin parallel beams which straddle the sheet S.

Means are provided for focusing and aligning the collimating means. A collar 119 secured to the front of the housing 112 by screws 121 is tapped at three or four circumferentially spaced points to receive adjusting screws which bear on a collar 122 secured in selected position on the front end portion of the tube 126 by a set screw 124. The rear end portion of the tube 126 is supported in an aperture in a partition in such manner as to permit slight movement of the front end of the tube laterally or vertically by the adjusting screws 120 to aim the light beams accurately toward the light receiving unit. A set screw 127 in the partition 125 releasably holds the rear end of the tube 126. Loosening the set screws 124 and 127 permits longitudinal movement of the tube to focus the lens 116. The diaphragm 114 is held between a sleeve 115 and a spring 128 which acts between the diaphragm and the lens retainer 117, and is selectively positioned by sliding the sleeve 115 longitudinally in the tube 126 and locking it by a set screw 129. The cap 118 is rotatable on the tube 126 to position the slits 164a and 19412 parallel to the yarn sheet and is frictionally held in selected position.

FIG. 10 shows in greater detail the light receiving unit 3. A housing 130 resting on base member 111 contains the light detection means 110 and beam converging means 198. The beam converging means N8 comprises a tube or sleeve 132 having a lens retainer 134 at its front end, closest to the edge of the sheet S, a second lens retainer 136 at its other end and a diaphragm 137 interposed between the lens retainers. A light compression spring 138 holds the diaphragm 137 in place against the end of a sleeve 139 interposed between the diaphragm and the lens retainer 136. A cap or hood 141) having parallel slit apertures 1116a and 1l6b is fitted over the front end of the lens sleeve 132 to transmit beams B1 and B2 to the beam converging means 108.

A lens unit 142 in the lens retainer 134 converges the beams of light which pass through the slits of the cap 140 and focuses the light of the beams to an image of the filament of light source of the lamp 17 in the plane and within a slit 144 in the lens retainer 136. The light passes through an aperture in the diaphragm 137 which blocks stray light rays. The slit 144 is preferably oriented parallel to the slits 106a and 10612 so as to receive the light images of the beams B1 and B2 focused by the lens unit 142 and to exclude the passage of any extraneous light. A diverging lens unit 146 mounted in the lens retainer 136 spreads the light passing through the slit 144 and directs it on the active element 110a of the light intensity detection means 110. The spreading of the light increases the life of the phototube. However, a converging lens, not shown, may be used to accomplish the same effect if the distance to the phototube is increased. The detection means 110 is preferably a photomultiplier or other phototube which is highly sensitive to small changes in light intensity.

The lens retainer 134 is axially slidable in the tube 132 to focus the lens 142 on the slit 144 of the lens retainer 136 and is locked in selected position by a set screw 148. The forward end of the tube 132 is adjustable vertically and laterally to align the optical system of the beam converging means 108 with the beams B1 and B2 by means of a plurality of circumferentially spaced screws 150 which extend through threaded holes in a collar 151 fixed on the forward wall of the housing 130 and bearing on a collar 152 locked to the tube 132 by a set screw 154. The rear end portion of tube 132 extends through an aperture 156 in a partition 157 of the housing 130 and is supported by a narrow lip portion 158 of the aperture which serves as a fulcrum to permit angular adjustment of the tube 132 by means of the aligning screws 151 The tube 132 is further held by a set screw 160 in the partition 157. When set screws 154 and 160 are loosened, the tube 132 is slidable axially'to adjust its position relative to the phototube 116). The cap 140 is rotatable so as to align its slit apertures 106a and 1116b parallel to apertures 104a and 1041) in the cap 118 of the light transmitting unit and is frictionally held in adjusted position.

It will be noted that the light receiving unit 3 is similar in construction to the light transmitting unit 2. While FIG. 11 is a front view of the light transmitting unit 2', the front of the light receiving unit 3' is similar.

The output of the phototube 110 is connected to a suitable output circuit, for example, a cathode follower 162 and suitable amplification means (not shown) to amplify the signals transmitted by the phototube to a level usable for actuating a stop-motion, operating visual or audio signals or other device.

It will thus be seen that this second embodiment of the invention also provides simple and effective apparatus for scanning both sides of a moving sheet of material by employing two light beams transmitted from a light transmitting unit positioned at one edge of the moving sheet and received by a light receiving unit positioned at the other edge of the moving sheet.

It will be understood that although the invention has been illustrated and described as apparatus for detecting a break that occurs in any of the yarns or strands of a sheet the apparatus, according to the invention, is equally usable to detect defects along a normal to the sheet, for example, knots, loops, fluff-balls and the like. The amplitude or diameter of the defects to be sensed or detected is determined by the spacing of the beams of flux relative to the sheet of material being inspected.

While two preferred embodiments of the invention have been illustrated in the drawings and particularly described, it will be understood that the invention in no way is limited to these embodiments and that modifications may be made within the scope of the appended claims.

What We claim and desire to secure by Letters Patent 1. Photodynamic monitor apparatus for photoelectrically inspecting a moving sheet of spun yarns, the apparatus having a working zone, in which said sheet material is disposed and through which it moves, in combination, a support, a source of luminous flux mounted on said support, means receptive of said luminous flux including optical means for directing two substantially parallel beams of luminous flux across said zone transversely across said sheet in proximity to opposite faces of said sheet material and substantially parallel to said opposite faces and along paths substantially in a common plane, means disposed to receive the total luminous flux of both said beams in said zone comprising a light-sensitive device responsive to luminous flux variations of said total fiux for generating an electrical signal in response to any diminution in the amount of the luminuous flux of both of said beams in said working zone, whereby any decrease of the luminous flux of either and both of said beams in said zone by said spun yarn causes said device to generate said signal, proportionate in amplitude and frequency to the variations in the diminution in the total amount of luminous flux in both of said beams.

2. Photo-dynamic monitor apparatus for photoelectrically inspecting a moving sheet of spun yarn, the apparatus having a working zone, in which said sheet material is disposed and through which it moves, in combination, a support, a single source of luminous flux mounted on said support, means receptive of said luminous flux including optical means for directing two substantially parallel beams of luminous flux across said zone transversely across said sheet in proximity to opposite faces of said sheet material and substantially parallel to said opposite faces and along paths substantially in a common plane, means disposed to receive the total luminous flux of both of said beams, in said zone comprising a light-sensitive device responsive to luminous flux variations of said total flux for generating an electrical signal in response to any diminution in the amount of the luminous flux of both of said beams in said working zone, whereby any decrease of the luminous flux of either of said beams in said zone by spun yarn causes, said device to generate said signal, proportionate in amplitude and frequency to the variations in the diminution in the total amount of luminous flux in both of said beams, means to exclude the effect of stray and ambient luminous flux on said monitor comprising lens means in said means disposed for receiving the total flux of said two beams for focusing said two beams into a single image of said source of luminous flux, means defining an aperture in the focal plane of said lens means, and said light-sensitive device being disposed to have the'luminous flux impinge thereon after passing through said aperture.

References Cited in the file of this patent UNITED STATES PATENTS 2,670,651 Burns et al Mar. 2, 1954 2,753,464 Stone July 3, 1956 2,967,947 Flook Jan. 10, 1961 

1. PHOTODYNAMIC MONITOR APPARATUS FOR PHOTOELECTRICALLY INSPECTING A MOVING SHEET OF SPUN YARNS, THE APPARATUS HAVING A WORKING ZONE, IN WHICH SAID SHEET MATERIAL IS DISPOSED AND THROUGH WHICH IT MOVES, IN COMBINATION, A SUPPORT, A SOURCE OF LUMINOUS FLUX MOUNTED ON SAID SUPPORT, MEANS RECEPTIVE OF SAID LUMINOUS FLUX INCLUDING OPTICAL MEANS FOR DIRECTIONG TWO SUBSTANTIALLY PARALLEL BEAMS OF LUMINOUS FLUX ACROSS SAID ZONE TRANSVERSELY ACROSS SAID SHEET IN PROXIMITY TO OPPOSITE FACES OF SAID SHEET MATERIAL AND SUBSTANTIALLY PARALLEL TO SAID OPPOSITE FACES AND ALONG PATHS SUBSTANTIALLY IN A COMMON PLANE, MEANS DISPOSED TO RECEIVE THE TOTAL LUMINIOUS FLUX OF BOTH SAID BEAMS IN SAID ZONE COMPRISING A LIGHT-SENSITIVE DEVICE RESPONSIVE TO LUMINOUS FLUX VARIATIONS OF SAID TOTAL FLUX FOR GENERATING AN ELECTRICAL SIGNAL IN RESPONSE TO ANY DIMINUTION IN THE AMOUNT OF THE LUMINUOUS FLUX OF BOTH 